Techniques have been developed to perfuse isolated cow eyes with whole blood at physiological rates of flow and controlled physiological levels of pO2, pCO2, and pH, for extended periods of time. These new techniques will be used to continue present studies on the effects of changes in blood parameters on the diameters of retinal arteries, veins, and capillaries. In addition, the whole blood perfusion technique will enable us to investigate, for the first time, differential red blood cell affinity for oxygen in vivo, by comparing the effects of fresh blood and blood stored for various lengths of time. Information gained in the study will contribute to a better understanding of the physiological role of these blood parameters in the eye and, ultimately, a better understanding of vascular ocular diseases such as diabetic retinopathy. The experiments may also have important implications for blood storage operations, such as blood banks.